1. Field Of Invention
This invention pertains to improved assembly and performance of photovoltaic modules using single-step or few-step lamination processes. The modules manufactured using the methods of the present invention exhibit significant cost savings over the current state of the art due, in part, to the reduced number processing of steps, elimination of certain low-throughput steps, and easy automation capability associated with the methods disclosed.
2. Description Of The Related Art
Photovoltaic (PV) modules are large-area optoelectronic devices that convert solar radiation directly into electrical energy. They require good electrical and optical performance and, because of the low energy density of solar radiation, exceptionally low manufacturing and material costs to be competitive with other electrical-energy generating options. Most PV modules presently use discrete crystalline-silicon (c-Si) solar cells that are connected in an electrical circuit and encapsulated with a glass cover and polymer backsheet for environmental protection. While very successful, the basic design and assembly process of present c-Si PV modules are over 20 years old and they exhibit certain drawbacks. The most commonly used module design inherently results in obscuration of a portion of the collecting surfaces of the solar cells, and the assembly process includes difficult steps requiring delicate and costly manipulation of components.
Existing uses and construction methods for photovoltaic cells and modules are described extensively in the literature. Useful references include the following: A. Schoenecker, et al., "An Industrial Multi-Crystalline EWT Solar Cell with Screen Printed Metallisation", 14.sup.th European Photovoltaic Solar Energy Conference and Exhibition (ECPVSEC), Barcelona, Spain, June/July 1997; D. Thorp, "Methods of Contacting Multijunction Silicon Photovoltaic Modules", 14.sup.th ECPVSEC, Barcelona, Spain, June/July 1997; F. Jeffrey, et al., "PVMaT Improvements in Monolithic a-Si Modules of Continuous Polymer Substrates", CP394, NREL/SNL Photovoltaics Program Review, AIP Press, New York, 1997, pp. 451-461; J. Hanoka, et al., "Advanced Polymer PV System", CP394, NREL/SNL Photovoltaics Program Review, AIP Press, New York, 1997, pp. 859-866; M. Kardauskas, et al., "Market-Driven Improvements in the Manufacturing of EFG Modules", CP394, NREL/SNL Photovoltaics Program Review, AIP Press, New York, 1997, pp. 851-858; G. Pack, et al., "New Component Development for Multi-100 kW Low-Cost Solar Array Applications", IEEE, 1982; K. Mitchell, et al., "The Reformation of Cz Si Photovoltaics", First WCPEC, IEEE, 1994; J. Gee, et al., "Emitter Wrap-Through Solar Cell", 23.sup.rd IEEE Photovoltaic Specialists Conference, Louisville, Ky., May 1993; J. Gee, et al., "Progress on the Emitter Wrap Through Silicon Solar Cell", 12.sup.th European Community Photovoltatic Solar Energy Conference, Amsterdam, The Netherlands, April 1994.
In a typical c-Si PV module manufactured using the current commercial technology, solar cells bearing electrical contacts on both the front and back surfaces are arranged in a grid and electrically connected either in series or in parallel. Most PV cells employed in commercial technology have electrical contacts on both the front and back surfaces on the cells to collect charges flowing through the semiconductor substrates of the cells. In order to connect the cells and create a power generating array, the front surface contacts of one cell are connected to the back surface contacts of another adjacent cell by means of electrical conductors (or tabs). Because of the electrical contact configuration of the cells and the necessity to string the cells electrically in a front-to-back fashion, the tabs on one cell necessarily overlay a portion of the collecting surface of that cell before connecting to the back contacts of an adjacent cell. Stringing of cells in this fashion has two important negative consequences for the light-to-electrical energy conversion efficiency of photovoltaic modules: 1) collection efficiency of the cells is not optimized due to a portion of their collecting surfaces being obscured by tabs, and 2) the packing density of solar cells within a module is diminished because of the space needed to accommodate the electrical connections going from the front of one cell to the back of an adjacent cell.
In the commercial process commonly used for module assembly using cells with both front and back contacts, several steps are required. Tabs are soldered on the front contacts of the cells individually, and then the cells are electrically connected by sequentially soldering them into the circuit. Next, being careful not to strain the electrical connections, cumbersome suction cup technology is employed to grasp the fragile electrical circuit and transfer it to an encapsulation work station. Finally, the cell circuit is encapsulated in the module. (See S. R. Wenham, M. A. Green, and M. W. Watt, Applied Photovoltaics, Chapter 5, Centre for Photovoltaic Devices and Systems, University of New South Wales, 1995.) This process typically requires at least three work stations with low throughput and relatively expensive automation. This 20-year-old module design and assembly process were adequate when the cost of silicon substrates completely dominated the cost of the finished PV module. However, recent advances in c-Si growth and wafering have reduced the cost of the wafer, and assembly is now the single largest cost element in a c-Si PV module. (K. W. Mitchell, et al., 1.sup.st World Conference on Photovoltaic Energy Conversion, 1266-1269,1994.)
These shortcomings associated with existing commercial PV module construction are overcome through the use of back contact c-Si solar cells and the assembly methods disclosed here. Briefly, the back-contact c-Si solar cells contemplated for use in the best mode for practicing the claimed invention are solar cells with coplanar contacts on the back surface which employ laser-drilled vias connecting the front-surface carrier-collector junction to an electrode grid on the back surface (see U.S. Pat. No. 5,468,652, James M. Gee). Use of these or other back-contact cells obviates the necessity for tabs to overlay the collecting surfaces of the cells, and enables manufacturers to arrange cells more closely together within the cell grid. Moreover, using back-contact cells can avoid the difficult automation and high stress points associated with front-to-back-lead attachment, and allow for planar processes that permit all of the cells in a PV module to be electrically connected in a single step.